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322,                      METALLURGY: OF IRON AND STEEL.
in accord with the evident .fact that a thin bar would cool f aster T than a round bar of somewhat less sectional area. The effect of: the finishing temperature, therefore, was to raise the tensile ';• strength of the flat more than it did the round, but not enough, to overcome the difference in .physical properties caused by the;,; shape of the bars.                                                                      . :
The reduction of area - is less in the case of. the flat, and the difference is more marked in the annealed than in the natural' bars. The elongation is higher in both kinds of flats than in the corresponding rounds, but the difference is greater in-the natur.a.1 bars. This appears, at first sight, to be an exception, but a decrease in gain is equivalent to a loss, and this brings it in accord with the decrease in the ductility, as shown.by the lessened ^reduction of area. The net result is as follows :„..
(1)   Flat bars differ from rounds in having less tensile strength, lower elastic limit, lower elastic ratio, greater elongation, and a slightly lower reduction of area................;
(2)   This difference is caused not by reason of a different finishing temperature, but in spite of it.
SEC. XVIh.—Comparative properties: of rounds of different diameter.—The variation in strength of bars is not confined to pieces of different shape, for it will exist in rounds,o.f 'different diameters. In Table XVI-K are given the results on a number of rivet rods where several tests were made from the same heat.: All the charges were of the same quality of steel, ranging from .11 to .15 per cent, in carbon, .02 to .04 per cent, ih phosphorus,; and .022 to .038 per cent, in sulphur..                                       r!
The number of heats would not be sufficient to justify a generali conclusion if there were only a single bar of each heat, but each; figure is the average of from 4 to 16' determinations.    In the comparison  of the  three-quarter  and,  seven-eighth .inch rounds there were 112 tests of the smaller size and 94 of the larger,) while in the comparison of the five-eighth and .three-quarter inch there were 32 tests of the former and 34 of the latter.   No average is given where less than four tests were taken of -the same. size from the same heat.    Comparing the seven-eighth inch with the three-quarter inch' bars, it will be found that in the' larger size the following changes occurred:                         ' • =••     :        !
• (1) The ultimate strength was lowered in ten heats and raised^